1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle driven by an engine and a motor, and in particular, relates to a control system of a hybrid vehicle capable of recovering the charge-discharge balance of the motor while travelling under conditions in which the power storage unit is liable to be excessively discharged.
2. Description of the Related Art
Conventionally, hybrid vehicles, which carry motors as power sources for driving in addition to engines, are known. Hybrid vehicles are divided into series hybrid vehicles and parallel hybrid vehicles. In the parallel hybrid vehicles, the motor connected to the engine assists the rotation of the drive shaft of the engine while charging a power storage unit using the motor as a generator. In parallel hybrid vehicles, a variety of control operations are executed so that the motor assists the engine at the time of acceleration and the power storage unit such as a battery or a capacitor is charged by deceleration regeneration at the time of deceleration, in order to maintain sufficient electric energy in the power storage unit including capacitors or a batteries (hereinafter, called a remaining charge, a xe2x80x9cstate of chargexe2x80x9d, or xe2x80x9cremaining capacitor or battery chargexe2x80x9d) to meet the driver""s demands. Specifically, because a high rate of deceleration regeneration is obtained after high speed cruising, the power storage unit regains a part of the consumed energy when decelerating. After the vehicle goes up a slope such as a mountain path, the vehicle can charge up the power storage unit by deceleration regeneration when the vehicle goes down the slope (as disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 7-123509).
In general, conventional hybrid vehicles are often quickly accelerated, then decelerated, and quickly accelerated again. In this situation, the vehicle cannot obtain sufficient deceleration regeneration. A hybrid vehicle, after going up a slope, may often run on level ground. In the former case, the remaining charge is decreased as the vehicles runs because regeneration is not sufficient. In the latter case, unless the vehicle goes down a slope, the vehicle cannot regain the charge of the power storage unit consumed when going up the slope.
In addition, when a hybrid vehicle uses a capacitor, which has a lower storage charge than that of a battery, a problem arises in that, because of a small reserve in the remaining charge of the capacitor, the vehicle is liable to be subject to engine stop and the fuel efficiency is likely to deteriorate.
In order avoid such a problem, a control device of the vehicle controls charging of the motor in line with a threshold value of the remaining charge while monitoring the remaining charge of the power storage unit. However, when the threshold value is set at a higher remaining charge, it is likely that energy of the power storage unit cannot be recovered. In contrast, when the threshold value is set at a lower value, charging is often executed, causing deterioration of the fuel efficiency, or causing a situation that energy generated by deceleration regeneration cannot be recovered because the remaining charge is maintained at a higher level.
It is therefore an object of the present invention to provide a control device for a hybrid vehicle, capable of managing the remaining charge of the power storage unit at an optimum state when the power storage unit is liable to be excessively discharged and when it is determined that the remaining charge of the capacitor should be recovered.
The first aspect of the present invention provides a control device for a hybrid vehicle provided with an engine and a motor, at least one of which is used as a driving source, and a power storage unit for storing energy generated by the output from said engine and regenerative energy produced by regeneration by said motor when said vehicle decelerates, comprising, a depth of discharge detecting device for detecting a depth-of-discharge of said power storage unit, a depth-of-discharge threshold value setting device for setting a threshold value of the depth-of-discharge of said power storage unit based on a value associated with the kinetic energy of the vehicle from an initial depth-of-discharge at the time of starting of the vehicle; and a charge control device for charging said power storage unit when the depth-of-discharge of said power storage unit exceeds said threshold value of the depth-of-discharge.
According to the first aspect, when the depth-of-discharge of the power storage unit detected by the depth-of-discharge detection device exceeds a threshold value set by the depth-of-discharge threshold setting device for setting based on the vehicle speed, it becomes possible to charge the power storage unit when the depth-of-discharge exceeds a predetermined threshold value so that the fuel consumption can be effectively reduced by preventing unnecessary charging.
In the control device for the hybrid vehicle according to the second aspect, said value associated with the kinetic energy of the vehicle is represented by a vehicle speed.
In the control device for a hybrid vehicle according to the third aspect, said charge control device comprises a charge setting device for setting the charge based on the value associated with the vehicle speed when said depth-of-discharge exceeds the threshold value.
According to the third aspect, when the depth-of-discharge exceeds a threshold value, the present invention is capable of charging the power storage unit for the charge set by the charge setting device. In addition, considering that the regenerative energy increases as the vehicle speed increases, it is possible to set the charge to an increased amount as the vehicle speed increases and to set the charge to a decreased amount and to increase the charging frequency as the vehicle speed decreases so that the capacitor is charged effectively according to the vehicle speed.
In the control device for a hybrid vehicle according to the fourth aspect, when charging said power storage unit, said charge control device further comprises a charge increasing device for increasing the charge to a higher value than the charge after the depth-of-discharge exceeds the threshold value than that before the depth of discharge exceeds said threshold value.
According to the fourth aspect, the depth-of-discharge of the power storage unit after exceeding the threshold value can be increased to a higher value than that before exceeding the threshold value by the charge increasing device, it is possible to improve the fuel efficiency by preventing unnecessary charging in the high vehicle speed region, and it is also possible to reduce the likelihood of the engine stopping by preventing a decrease of the charging frequency in the low vehicle speed region.
In the control device for a hybrid vehicle according to the fifth aspect, while controlling the charge by the charge control device, said control device of the hybrid vehicle comprising a motor drive limit control device limits the driving of the vehicle by the motor.
According to the fifth aspect, while controlling the charge by the charge control device, it is possible to prevent the power storage unit from being discharged by limiting the motor drive by the motor drive limit control device, and the power storage unit can be managed without feeling concern of the over-discharge.
In the control device for the hybrid vehicle according to the sixth aspect, said motor drive limit control device corresponds to a determination threshold value modification device which modifies the motor drive determination threshold value based on the driving state of the vehicle so as to make it less likely to drive the vehicle by the motor.
According to the fifth aspect, since said motor drive limit control device, that is, the determination threshold modification device makes it possible to modify the motor drive determination threshold value so as to make it less likely to drive the vehicle by the motor, the frequency of the motor drive is reduced, this aspect of the present invention is effective in that the power storage unit can be recovered rapidly.